In recent years, an apparatus conforming to an MPEG (Moving Picture Experts Group) scheme and the like is becoming widespread in both broadcast stations which distribute information and ordinary households which receive information, where the MPEG scheme treats image information as digital and compresses the information, for the purpose of efficiently transmitting and storing information, by means of an orthogonal transform such as a discrete cosine transform and motion compensation while exploiting redundancy specific to the image information.
MPEG-2 (ISO (International Organization for Standardization)/IEC (International Electrotechnical Commission) 13818-2), in particular, is defined as a general-purpose image encoding system that is a standard covering both an interlaced scan image and a progressive scan image as well as images with a standard resolution and a high resolution. The MPEG-2 is now used widely in a wide range of applications for use by professionals and consumers. The use of the MPEG-2 compression scheme can realize a high compression ratio and satisfactory image quality by assigning a code amount (bit rate) of 4 to 8 Mbps to an interlaced scan image with a standard resolution having 720×480 pixels, or 18 to 22 Mbps to an interlaced scan image with a high resolution having 1920×1088 pixels, for example.
The MPEG-2 has targeted high image-quality encoding adapted mainly for broadcasting, but has not been suited for an encoding system with a code amount (bit rate) lower than that of MPEG-1, namely, an encoding system with a higher compression ratio. The need for such encoding system is expected to increase in the future as a mobile terminal is coming into wide use. In response, an MPEG-4 encoding system has been standardized. With regards to the image encoding system, the standard was approved as an international standard ISO/IEC 14496-2 in December, 1998.
Furthermore, a standard named H.26L (ITU-T (International Telecommunication Union Telecommunication Standardization Sector) Q6/16 VCEG (Video Coding Expert Group)), which is initially intended for encoding an image used in a television conference, has been in the process of becoming standardized in recent years. While requiring a greater amount of calculation in encoding and decoding compared to the conventional encoding systems such as the MPEG-2 and the MPEG-4, the H.26L is known to achieve higher encoding efficiency. As a part of the actions pertaining to the MPEG-4, moreover, a standardization achieving yet higher encoding efficiency by using the H.26L as a base and incorporating a function not supported in the H.26L has currently been in the process as Joint Model of Enhanced-Compression Video Coding.
The standard became an international standard in March, 2003 under the name of H.264 and MPEG-4 Part 10 (Advanced Video Coding; hereinafter noted as AVC) regarding the standardization schedule.
Further, as an extended activity, the standardization of FRExt (Fidelity Range Extension) including a coding tool necessary for business use such as RGB, 4:2:2, or 4:4:4, 8×8DCT (Discrete Cosine Transform) and quantization matrix stipulated by MPEG-2 has been completed in February, 2005, whereby AVC can be used as an encoding system capable of suitably expressing even film noise included in movies, and has come to be employed for a wide range of applications such as Blu-Ray Disc (registered trademark) and the like.
However, nowadays, needs for further high-compression encoding have been increased, such as intending to compress an image having around 4000×2000 pixels, which is quadruple of a high-vision image, or alternatively, needs for further high-compression encoding have been increased, such as intending to distribute a high-vision image within an environment with limited transmission capacity like the Internet. Therefore, with the above-mentioned VCEG (Video Coding Expert Group) under the control of ITU-T, studies relating to improvement of encoding efficiency have been continuously performed.
However, it was concerned that a macroblock size of 16 pixels×16 pixels was not optimal for a large picture frame provided in UHD (Ultra High Definition; 4000 pixels×2000 pixels) which is to be the target for a next-generation encoding system.
Accordingly, a JCTVC (Joint Collaboration Team-Video Coding) that is a joint standardization group formed by the ITU-T and the ISO/IEC is currently working to standardize an encoding system called an HEVC (High Efficiency Video Coding) for the purpose of further improving the encoding efficiency achieved by the AVC (refer to Non-Patent Document 1, for example).
The HEVC encoding system defines a coding unit (CU) as a processing unit similar to the macroblock used in the AVC. Unlike the macroblock used in the AVC, the size of the CU is not fixed to 16×16 pixels but specified within image compression information in each sequence.
By the way, in HEVC, an encoding system called most probable mode (MPM) can be used. In this mode, the encoder includes, in the stream, a flag mpm_flag that specifies whether the MPM is used for a PU of interest (prediction unit) to be processed and mpm_lr_flag that indicates which intra prediction direction of adjacent PU is applied to the PU of interest.
In contrast, when the MPM is not used, the encoder includes intra_dir_mode that indicates a prediction direction of intra prediction of the PU of interest in the stream.
The decoder decodes the stream, and decodes encoded data by a decoding method corresponding to the encoding system based on the information.